The present invention relates in general to pistons and, more particularly, the invention relates to a piston assembly which reduces the consumption of a lubricating agent circulated to provide lubrication between the outer sidewall of such piston assembly and the interior sidewall of a bore in which such piston assembly is mounted for reciprocal movement. The volume of a lubricating agent such as oil that passes the compression rings and eventually works its way to the top surface of the piston head is normally controlled through different ring designs and ring loadings against the adjacent cylinder walls. Because such lubricating agent is normally oil, an oil ring is provided on the outer surface of the piston which serves the purpose of coating the cylinder wall with an oil film. The compression ring or rings are provided in order to seal the variable volume chamber to prevent the fluid, which is either compressed or combusted, from entering the crankcase. In addition, the compression rings supposedly will scrape away the lubricating oil that has been wiped into the cylinder sidewall by the oil ring or rings.
It was believed that this configuration would normally provide satisfactory oil passing in a fluid compressor, for example, however, it has been found to be less than satisfactory under a variety of conditions of operations in certain types of fluid compressor exhausters. It had been thought up until the present time that passing of such lubricating oil could be controlled in an efficient manner through the use of a vacuum maintaining valve in the system in order to maintain a crankcase volume of between about 20 to 21 inches of Hg. vacuum. This was believed to be very sound reasoning because the pressure differential would always be toward the crankcase. Nevertheless, review of a number of oil passing tests that have been conducted over an extended period of time and under a number of operating conditions have shown a sensitivity to the main reservoir vacuum which will influence the maximum level of crankcase vacuum that can be achieved and also determines the magnitude of the pressure differential that is developed across the compression rings.
Furthermore, the inspection of a number of vacuum cylinders in current ongoing production tests have likewise shown that an excessive amount of lubricating oil is being carried past the compression rings to the top surface of the piston. In some of the more severe cases, rather large puddles of oil have been observed on the top surface of the vacuum pistons in the fluid compressor. The observation suggests that all of the oil that is being passed does not pass by the sealing surface of the compression ring as one would expect but such ol may possibly migrate behind the compression ring until such time as it enters the top of the piston and is forced out through the valves to the atmosphere. In any event, this situation can not be tolerated because the characteristically high oil passage on such vacuum cylinders serves to increase the maintenance requirements in view of the fact that the oil will cause carboning of the valves under a high discharge temperature condition. When carboning of the valve occurs, which has sometimes been reported in as short a period as three months, the compressor must be taken out of service so that the valves can be cleaned.